contact point is dead center of the nock. All nocks fall in this category regardless of make. There are very very very few differences in nocks on the market these days when you cut the OAD in half you're talking thousandths of an inch. Take into account MOST I'd bet 90% or more of folks are shooting similar nock profiles (regardless of make here).
Ie a 5/16 classic vs a 5/16 snap on is going to be almost exactly or within a few thou over half the diameter. Maybe 10 thou but I doubt it. Remember we're only concerned with HALF of the diameter when it comes to the nock. ANd that's assuming nock size is the same or smaller than shaft size. Most nocks are smaller than shaft sizes these days, I haven't seen one that is the same size at the throat since the stotler mids. EVERY nock is going to contact on the dead center point, so everything below is void relative to nock over all nock heigth. I still have my old mercs and stotler mids and those are slightly tapered if I remember right. And even with the stotlers they may push the nock up a tad more than a bohning classic but 1/16" of an inch it wont do. MAYBE 1/32...maybe!
I'd guess less 1/64, or less! over all..
NOW....
IF you are making nocks say horn nocks or glued on self nocks etc. Than sure its possible that the diameter is going to play some roll that is going to be noticeable in your over all nock placement. Put some large ears on for shooting a large nock so you can shoot Turkish style, again a pinch style release..than again yes its possible. Pinch like Lars is doing, not thumb rings like Yumi's.
But reality is unless you're putting a tree on the back end its not likely going to have any or very very minimal effect. I'd bet there is less than 1% of guys using nocks larger than the diameter of the shaft at the throat, and again 1% of folks shooting same diameter nocks at the throat as the same diameter of the shaft. and I'd also make the guess that 95% of those shooting shot some form of tapered nock like a classic, snap on, old merc, etc.
So take a guy shooting a tapered or barrel tapered 11/32 shaft with a 5/16 nock. 1/4 over is not out of the question. half of 11/32 is .172, which is less than 1/4 at .25. Half of 5/16 is .156. These numbers are based on the ideal that the nock is the same diameter of the shaft. Nock off a few more though for the tapered flats where your nock sits and it can be even lower yet.
SO its POSSIBLE to see a lower than normal nock height on some setups. Take in other factors, tiller, bow design, rest material/placement (not everyone shoots off the shelf), etc etc... IT IS POSSIBLE and I personally have proven it! That doesn't mean its NORMAL! I agree 100% it is unlikely you'll see anything below 1/4 very often....or even at 1/4.
I've also seen guys who measure with thick rests, like seal skin, or unsheared Velcro or a host of others, make references to those lower nocks. Guys shooting leather or a hard flipper style are able to get 100% accurate numbers. This is where my 1/8 figure came from, a bow shot (not mine personally), with a high material. He was a pretty good buddy of mine so we had this very conversation over a lot of years.
My current bow is like most at 3/8. But in 25 years I've shot and owned a LOT of sticks that didn't fit any mold. A Robertson longbow in particular still to this day bugs the crap out of me! Great bow, didn't fit my shooting style!
now....in the end does it really matter? NO!
The only thing that matters is arrow flight! that's it, its really that simple. If it flies so be it, should it or should it not is a good campfire discussion or range fun time, semantecs are just that, hypothetical equations. I enjoy all of it.